The present invention relates to a circuit for preventing rupture of a device caused by overcurrent, in a lighting circuit for a discharge lamp incorporating a DC-DC converter. The circuit limits an electric current of a switching device of the converter.
A lighting circuit for a discharge lamp, such as a metal halide lamp, is known which incorporates a DC power source, a switching power source circuit, a DC-AC converting circuit and a start circuit. The switching power source circuit configured as a DC-DC converter circuit is controlled by, for example, a known PWM (Pulse Width Modulation) method.
The conventional method is arranged to control change in the ON/OFF ratio or the duty ratio (the duty cycle) with respect to a semiconductor switching device constituting the converter circuit so that change in the output voltage is permitted. As a reliable countermeasure to prevent rupture because of overcurrent in the switching device in the converter circuit, a so-called pulse-by-pulse current limiting method is known.
FIG. 11 shows an essential portion of an example 71 of a circuit structure incorporating a DC power source 72, a capacitor 73, a transformer T (only a primary winding 74 is illustrated), a FET (Field Effect (type) Transistor) 75 serving as a switching device, and a current detecting resistor 76. That is, the capacitor 73 is disposed in parallel with the DC power source 72. An end of the primary winding 74 of the transformer T is connected to the positive terminal of the DC power source 72. Another end of the transformer T is connected to the drain of the FET 75. The source of the FET 75 is connected to a negative terminal of the DC power source 72 through the current detecting resistor 76. The gate of the FET 75 is supplied with a control signal from a control circuit (for example, a circuit incorporating a PWM control IC)(not shown). In response to the signal, the ON/OFF operation of the FET 75 is controlled.
FIG. 12 schematically shows waveforms of electric current xe2x80x9cIxe2x80x9d which flows in the FET 75 and a control signal xe2x80x9cSDxe2x80x9d for the FET (period xe2x80x9cTonxe2x80x9d indicates ON-period of the device and xe2x80x9cToffxe2x80x9d indicates OFF-period of the device). The voltage of the current I is converted and detected by the current detecting resistor 76 (that is, when the resistance value of the current detecting resistor 76 is xe2x80x9cRxe2x80x9d, detected voltage value is xe2x80x9cIxe2x80xa2Rxe2x80x9d). The detection signal is transmitted to the control circuit.
The level xe2x80x9cVlimxe2x80x9d indicated with a dashed line shown in FIG. 12 indicates a level (an upper limit) to which the electric current is limited. When the current I reaches this level, the FET is brought to the OFF-state. That is, as the level Vlim is set to a higher level, the length of the OFF-period Toff is shortened. When the level Vlim is set to a lower level, the length of the OFF-period is elongated. As a result of this control, the current value is limited to prevent flow of overcurrent in the switching device (FET). As a result, heating and rupture of the device are prevented.
The circuit having the foregoing current limiting function, however, requires the current detecting resistor 76 to have a relatively large capacitance (for example, a resistor device of a several-W class). Thus, there arises a problem in that a reduction in size and cost of the apparatus cannot be achieved. A similar problem arises when a current transformer for current/voltage converting is employed without use of the current detecting resistor 76.
Accordingly, an object of the present invention is to easily realize current limitation of a switching device, in a DC-DC converter of a lighting circuit for a discharge lamp, and to reduce the size and cost of the associated circuit apparatus.
According to the present invention, there is provided a lighting circuit for a discharge lamp comprising: a DC-DC converter for converting DC voltage; and, a control circuit for controlling output voltage from the converter by controlling ON/OFF of a field effect transistor that serves as a switching device in the converter, wherein an electric current which flows in the field effect transistor is monitored by detecting a voltage between a drain and a source of the transistor, the control circuit turning the field effect transistor off when the voltage value is not smaller than a reference value.
The present invention has a structure that provides for monitoring the electric current which flows in the field effect transistor by detecting voltage between the drain and the source. When the voltage value is not smaller than a reference value, the field effect transistor is turned off to limit the ON-period. Thus, rupture of the device because of overcurrent can be prevented. Moreover, a current detecting resistor or the like having a large capacitance, for detecting the electric current w flows in the field effect transistor, is not required.